Organic light emitting diodes (OLEDs) typically comprise several layers of organic materials interposed between conductive thin film electrodes, with at least one of the organic layers being an electroluminescent layer. When a voltage is applied to the electrodes, holes and electrons are injected from the anode and cathode, respectively. The holes and electrons injected by the electrodes migrate through the organic layers to reach the electroluminescent layer. When a hole and an electron are in close proximity, they are attracted to each other due to the Coulomb force. The hole and electron may then combine to form a bound state referred to as an exciton. As is well known, an exciton may decay though a radiative recombination process, in which a photon is released. Alternatively, an exciton may decay through a non-radiative recombination process, in which no photon is released.
A radiative recombination process can occur as a fluorescence or phosphorescence process, depending on the spin state of the electron-hole pair (i.e. the exciton). Specifically, the exciton formed by the electron-hole pair may be characterized as having a singlet or triplet spin state. Generally, radiative decay of a singlet exciton results in fluorescence, whereas radiative decay of a triplet exciton results in phosphorescence.
Approximately one quarter of excitons formed in organic materials typically used in OLEDs are singlet excitons, with the remaining three quarters being triplet excitons. As is well known, a direct transition from a triplet state to a singlet state is considered to be a “forbidden” transition in quantum mechanics and, as such, the probability of radiative decay from a triplet state to a singlet state is generally very small. Unfortunately, the ground states of most organic materials used in OLEDs are singlet states, which prevent efficient radiative decay of an exciton in a triplet state to a singlet ground state at ambient temperatures in these materials. As such, in typical OLEDs, electroluminescence is primarily achieved by fluorescence, therefore giving rise to a maximum internal quantum efficiency of about 25%. It is noted that, as used herein, internal quantum efficiency (IQE) will be understood to be the proportion of all electron-hole pairs generated in the device which decay through a radiative recombination process.
Although radiative decay from a triplet state to the ground singlet state occurs at an extremely slow rate in most organic materials, the rate of decay (i.e. recombination rate) may be significantly increased by introducing species having high spin-orbit coupling constants. For example, complexes of transition elements such as Ir(III) and Pt(III) have been employed in so-called phosphorescent OLEDs, as the high spin-orbit coupling constants of these species promote a more efficient radiative decay from a triplet state to the ground singlet state. As such, some or all of the approximately 75% of excitons in the triplet states may also transition efficiently to the singlet ground state and emit light, thus resulting in a device having a maximum IQE of close to 100%.
The external quantum efficiency (EQE) of an OLED device may be defined as the ratio of charge carriers provided to the OLED to the number of photons emitted by the device. For example, an EQE of 100% implies that one photon is emitted for each electron that is injected into the device. As will be appreciated, the EQE of a device is generally substantially lower than the IQE of the device. The difference between the EQE and the IQE can generally be attributed to a number of factors such as absorption and reflection of light caused by various components of the device. One way of enhancing the EQE of a device is to use a cathode material that has a relatively low work function, such that electrons are readily injected into the adjacent organic layer during the operation of the device. Typically, aluminum is used as the cathode material due to its favourable electrical and optical properties. Specifically, it has a work function of 4.1 eV, is an excellent conductor, and has a relatively high reflectance in the visible spectrum when deposited as a film. Moreover, aluminum has advantageous processing characteristics compared to some other metals. For instance, aluminum has a deposition temperature of approximately 1600° C.
Although aluminum is typically chosen as a cathode material, in some applications, magnesium may, on its face, be a more favourable cathode material than aluminum. When compared to aluminum, magnesium has a lower work function of 3.6 eV. Magnesium can also be thermally deposited at deposition temperatures, for example, of 400° C. or less, which is substantially lower than the deposition temperature of aluminum, and is therefore more cost effective and easier to process.
However, as is noted in U.S. Pat. Nos. 4,885,211 and 5,059,862, substantially pure magnesium could not be used as an effective cathode for organic optoelectronic devices, since its adhesion to organic materials is poor and its environmental stability is low. US Publication No. 2012/0313099 further describes magnesium's poor adhesion to organic surfaces. Additionally, magnesium is prone to oxidation and, as such, devices with magnesium cathodes are difficult to manufacture and operate under oxygen and/or humid environments since the conductivity of the cathode quickly deteriorates as magnesium oxidizes.
Although it is possible to deposit magnesium on various inorganic surfaces such as those of glass and silicon, the sticking coefficient of magnesium on these surfaces is generally low. As such, the deposition rate of magnesium on such surfaces is also relatively low thus typical deposition processes known in the art are generally not cost-effective.
In U.S. Pat. No. 6,794,061 to Liao et al., an organic electroluminescent device is provided that includes an anode, a substantially pure magnesium cathode, an electroluminescent medium disposed between the anode and the cathode, and an adhesion-promoting layer in contact with the cathode and the electroluminescent medium, wherein the adhesion-promoting layer comprises at least one metal or metal compound. However, at least some metals or metal compounds suggested for use as adhesion-promoting layers by Liao et al. may be unstable and therefore not suitable for long-term use in many applications. For example, metals such as cesium are known to be strong reducing agents, and as such, they quickly oxidize when exposed to water, humidity, or air. Therefore, deposition of such metals is often complicated and difficult to integrate into conventional manufacturing processes for producing organic optoelectronic devices.
It has also previously been reported that magnesium will selectively adhere to the coloured states of some photochromic molecules [JACS 130, 10740 (2008)]. However, the applications of this discovery in the context of organic optoelectronic devices are few, as these materials are not typically used in organic optoelectronic devices.
As such, there exists a need for a method for promoting adhesion of magnesium to a surface that alleviates at least one of the deficiencies known in the art.